Nor were the besiegers greatly elated; the tiny Fort of St. Elmo had delayed them for five weeks and had cost them 8,000 men and their best general.  The Order had lost 1,300 men, of whom 130 were Knights, and the disparity of the losses shows the impatience and recklessness of the Turkish attacks.

Mustapha now transferred the main part of his army to the other side of the Grand Harbour, and, drawing a line of entrenchments along the heights on its eastern side, succeeded in investing completely the two peninsulas of Senglea and Il Borgo.  Batteries were established and a constant bombardment commenced, the main target being Fort St. Michael at the end of Senglea, on which a converging fire was brought to bear.  Unable to bring his fleet into the Grand Harbour under the guns of St. Angelo, Mustapha had eighty galleys dragged across the neck of Mount Sceberras and launched on the upper waters of the Grand Harbour.  This was a blow to the besieged, as it meant an attack by sea as well as by land, and La Valette made all the preparations possible to meet the danger.  Along the south-west side of Senglea, where the beach is low, he constructed, with the aid of his Maltese divers, a very firm and powerful stockade to prevent the enemy galleys from running ashore, and he also linked up Il Borgo and Senglea with a floating bridge.

On July 15 the Turks delivered a grand assault by sea and by land.  The attack by sea, under the command of the renegade Candellissa, proved the more formidable.  At the critical moment the defenders were thrown into confusion by an explosion on the ramparts, during which the Turks were able to make their way through the stockade and into the fortress, being checked with difficulty by the desperate resistance of the garrison and finally driven out by a timely reinforcement sent by La Valette.  Ten boatloads of troops sent by Mustapha incautiously exposed themselves to the guns of St. Angelo and were almost all sunk, while the attack on the land side, led by Hassan, Viceroy of Algiers and son of Khaired-Din Barbarossa, proved an utter failure.

As at the siege of Rhodes, so at Malta, a distinct part of the fortifications had been allotted to each langue to defend.  The langue of Castile held the north-east section of Il Borgo, which was destined to be the scene of most desperate fighting.

On August 7 a joint attack was made on the land side of Senglea and on the bastion of Castile.  On that day the Turks came nearer success than ever before or after.  Mustapha’s desperate attacks on Senglea were at last successful:  masters of the breach made by their guns, the assailants’ weight of numbers began to tell, and slowly the defenders were being pushed back inside the fortress.  At this moment, to everyone’s amazement, Mustapha sounded the retreat.  The little garrison of the Citta Notabile, which had been left alone by the Turks, had been raiding the enemy’s lines as usual, and, hearing the grand assault was in progress, had made a determined attack on the Turkish entrenchments from behind, burning and slaying all they could find.  The confusion arising from this started the rumour that Sicilian reinforcements had landed and were attacking the Turkish army.  Mustapha, in fear of being surrounded, drew off his troops in the moment of victory.